Stirling Engines offer exceptional advantages of multifuel (including solar) capabilities, very low emissions with hydrocarbon fuels, exceptionally high thermodynamic efficiency, and the potential for very long low-maintenance operation. The major technical reason that these advantages have not been commercially realized is that available means for sealing the necessary pistons and rods (except at very low power levels) are unreliable and compromise performance and/or lifetime. The present invention offers a unique pressure balanced sealing mechanism which addresses all the technical problems with existing seals, is appropriate for all power levels of interest, and has the potential for cost effective implementation.
Rod seals and piston seals in a Stirling Engine typically consist of single or multiple sliding or scraping seals which are intended to allow cyclic volume change of the gaseous working fluid or to transmit mechanical power through a rod to a load while preventing egress of the gas or ingress of oil or oil vapor to the gaseous working fluid. The basic problem with such non-hermetic seals for long-life requirements in such machinery is that they cannot totally prevent loss of working fluid. Even more importantly, ingress of oil vapor to the engine working fluid contaminates heat transfer surfaces. Sliding seals also impose substantial friction losses. Use of water as a lubricant in Stirling Engine applications has been suggested, but this requires use of air rather than helium or hydrogen for the engine working fluid, which materially compromises engine performance.
An alternate sealing means which utilizes an elastomeric rollsock seal to effect sealing in a Stirling Engine was developed to an advanced state prior to discovery of this invention, but was abandoned because it was complex, expensive, and did not produce consistent lifetime results. The rollsock seal bears a superficial resemblance to the subject of the present invention, but major differences exist which cause the rollsock seal to be commercially impractical. An auxiliary gas space sliding seal must be used to isolate the rollsock seal from engine cyclic pressure variations, and a complex pressure regulator must be used to maintain a substantial static pressure difference across the rollsock seal in a specified direction. Due to its flexible nature, a rollsock seal must be made from elastomeric materials. The gas permeability of such materials limits the degree of sealing attainable with this seal structure. A true hermetic seal cannot be accomplished at the working gas pressures encountered in Stirling applications.
Metal diaphragms have been used as hermetic seals in Stirling machines, but the limited available stroke of a diaphragm make such seals impractical. This is particularly true in larger engines or heat pumps.
Metal bellows have previously been used to provide a hermetically sealed moving interface in small hydraulic output Stirling Engines for an artificial heart power source application. These seals have proven to be cost effective, reliable, long lived, and efficient in that application where power output requirements are limited, and are on the order of five watts. Attempts to scale that technology to the fifteen-kilowatt level showed that hydraulic flow losses were unacceptably high.
The prior bellows seal was applied only to a free-floating piston across one end of a supporting bellows, which is inherently self balancing. No effort was directed to the challenge of sealing a frictional clearance seal along a rod or piston so as to hermetically seal the working gas from liquid lubricants necessary for reciprocation of a power transmitting member so supported.
The sealing approach which is the subject of this disclosure represents the successful conclusion of an effort to retain the advantages of the heart engine bellows seal technology in a seal concept which is practical for high power output engines. Unique and innovative features have been combined to achieve these results.
Briefly, the present invention offers an improved method for sealing the working gas from a lubricating fluid along a clearance seal for a reciprocating working member in a Stirling Engine or Heat Pump. A long life hermetic seal is accomplished by means of one or more metal bellows. A critical feature which makes the bellows seal practical is provision of means for pressure balancing the bellows which maintains essentially equal instantaneous pressures in a gas and a liquid at opposite sides of each bellows used, even though the working gas pressures may vary cyclically over a range of hundreds of pounds per square inch at a rate of thousands of times per minute. The instantaneous pressure history may differ greatly among individual bellows, even when two bellows are used in the same sealing mechanism. Pressure balancing is accomplished by a judicious selection of effective bellows seal diameter relative to the associated rod or piston diameter, coupled with an auxiliary pressure compensator which may also actuate a pilot valve to provide for fluid makeup or overflow as needed to accommodate normal internal fluid leakage within one fluid region. The sealing means are described in various configurations which use either single or double bellows seals depending on availability of external hydraulic makeup fluid and which can function as either rod seals or piston seals for either kinematic or free-piston or hybrid (e.g., Ringbom) Stirling Engines or Heat Pumps.